Imaging system

ABSTRACT

An improved method for removing residual toner images from an electrostatic recording surface adapted for continuous automatic operation comprising charging residual toner images supported on an electrostatic recording surface to a first polarity, then cascading a two-component developer material including carrier beads and toner particles in triboelectric relationship over the residual toner images and recording surface and concurrently biasing said residual toner images with a potential at a second polarity when commencing a pass through the development zone, whereby the residual toner images are released from said recording surface and intermixed with said developer material for development of said recording surface while in the development zone.

United States Patent [151 3,640,707

Caldwell Feb. 8, 197 2 [54] IMAGING SYSTEM Primary ExaminerGeorge Fv Lesmes Assistant Examiner-M. B. Wittenberg [72] inventor: John P. Caldwell, Fairport, Attorney-James J Ralabate, Norman E. Schrader and Melvin I731 Assignee: Xerox Corporation, Rochester, NY. Klein [22] Filed: "0. ll, I969 ABSTRACT Appl No.: 884,106

An improved method for removing residual toner images from an electrostatic recording surface adapted for continuous automatic operation comprising charging residual toner images supported on an electrostatic recording surface to a first polarity, then cascading a two-component developer material including carrier beads and toner particles in triboelectric relationship over the residual toner images and recording surface and concurrently biasing said residual toner images with a potential at a second polarity when commencing a pass through the development zone, whereby the residual toner images are released from said recording surface and intermixed with said developer material for development of said recording surface while in the development zone.

1 Claims, 2 Drawing Figures PATENIEUFEB 8 1m 3.640.707

INVENTOR. JOHN F? CALDWELL Y MAM ATTORNEY IMAGING SYSTEM This invention relates to improved electrostatic imaging and more specifically to an improvement over the development of electrostatic latent images and the removal of the residual toner images from a support surface.

It is universally known that a commercially successful mode of development employed in automatic xerographic apparatus described in Walkup Pat. No. 2,618,551 comprises a developer generally consisn'ng of toner and a granular material called carrier, which by mixing triboelectrically acquire charges of opposite polarity, is gravitationally cascaded over the xerographic plate carrying the electrostatic latent image. Although carrier typically comprises spherical particles, in various other systems the carrier may be in various forms and substances including flat platelets, cubical solids, synthetic and natural fibers, metallic filing, and others. In addition to the cascade development system, magnetic brush, liquid developer, fluidized bed, powder cloud and other development systems are well known.

A commercially successful mode of cleaning employed in automatic xerographic apparatus is described in US. Pat. Nos. 2,751,616 and 2,832,977,wherein a brush with bristles which are soft and of suitable triboelectric characteristics, and yet sufiiciently firm to remove residual toner particles from the xerographic plate, is used to whisk residual toner images from the surface of the xerographic plate. In addition, webs or belts of soft fibrous materials or tacky materials, and other cleaning systems are known.

In spite of the successes that have been achieved in cleaning, the .prior art solutions to the problems in the development and cleaning steps in the xerographic process are not entirely satisfactory. For example, cleaning still typically requires bulky apparatus and a separate and distinct cleaning station. Experience has shown that the greater the number of apparatus stations necessary to carry out the xerographic process, the greater the danger of toner powder escaping throughout the mechanism and dusting the operating apparatus. Many cleaning systems typically require more than one pass through the cleaning station, requiring more time for cleaning the xerographic plate and thereby making the cleaning step one of the limiting factors in the operating time of the xerographic cycle. Also typically, development and cleaning must be performed at different areas of the xerographic plate, which requires more apparatus to ensure that the portion of the xerographic plate being used to reproduce the desired image is correctly registered at each of the xerographic stations. Experience in the art of photoconductors has shown that the greater .the number of passes necessary to clean or develop the surface of said photoconductor, the fewer the number of cycles through which said photoconductor or xerographic plate can be used with acceptable image quality. The surface of the photoconductor is partially abraded by multiply passed through development or cleaning steps, and scratches in the surface of the plate may mechanically pick up toner particles thereby darkening the background areas of desired images. In addition, increased numbers of passes through development or cleaning stations tend to increase toner consumption and to impair toner concentration in the development system. Each of these effects contributes to reduced image quality in the prior art systems.

Efforts to solve the above problems have led to new and different imaging systems, such as, the system for simultaneous development of electrostatic latent images and removal of residual toner images from the image support as described in copending application Ser. No. 789, 03l filed Dec. 31, 1968, in the names of Volkers et al. The present invention is especially intended to overcome the above disadvantages and be an improvement to the foregoing system.

It is, therefore, an object of this invention to provide an imaging system which overcomes the above-noted disadvantages and satisfies the above-noted needs.

It is also an object of this invention to provide a system for development of latent images and removal of residual images on a support surface.

It is another object of this invention to provide a system for the simultaneous development and cleaning of an electrostatic latent image support surface or electrostatographic surface.

It is another object of this invention to provide a system for the simultaneous development and cleaning of essentially the same area of an electrostatographic surface such as a xerographic plate.

It is another object of this invention to provide a system for the simultaneous development and cleaning at essentially the same area of a xerographic plate at the same station in a xerographic in a xerographic apparatus.

It is another object of this invention to provide a system for cleaning the surface of a xerographic plate in a single cleaning pass. I

It is another object of this invention to provide a system for improving image quality of copies made by the xerographic process.

It is yet another object of this invention to provide a system to increase toner efi'rciency in the xerographic process.

It is another object of this invention to provide a system to prolong the life of the photoconductive insulating layer of the xerographic plate, or any other exposed surface of the xerographic plate or support surface. 1

It is still another object of this invention to provide a system enabling more quiet operation of electrostatic copying apparatus.

It is still another object of this invention to provide a more simple and compact electrostatographic copying apparatus.

The foregoing objects and others are accomplished in accordance with this invention by providing a system, which when used in conjunction with an electrostatic latent image support surface provides for the substantially simultaneous removal of residual images comprising residual toner particles from said surface, and development of electrostatic latent images or charge patterns on essentially the same area of said surface.

For a better understanding of the invention as well as other objects and further features thereof, reference is made to the following detailed disclosure of the preferred embodiments of this invention taken in conjunction with the accompanying drawing in which:

FIG. 1 is a schematic view of xerographic apparatus incorporating the improved system of the present invention; and

FIG. 2 is an enlarged view of the upper portion of the xerographic plate and development zone shown in FIG. 1.

Referring now to FIG. 1 which discloses a xerographic apparatus showing the steps used in the xerographic process, but embodying the improved system of the present invention there is shown a drum 10 with photoconductor layer 11 adapted for rotation past processing stations. As the surface of the drum advanced during a xerographic cycle, a corona discharge device 12 initially charges said surface. The charged surface then advances through an exposure station 13 where the lightand-shadow image desired to be copied is projected onto the surface of the drum 10.

The charged and exposed surface of the drum now bearing the electrostatic latent image corresponding to the light-andshadow image projected thereon, then advances into the combination development-cleaning station 14 according to the present invention as will be described more frilly hereinafter. Development and cleaning are carried out by a cascade of developer comprising toner and carrier which develops electrostatic latent images and at the same time removes residual images comprising toner particles, typically adhering to the surface of the drum in essentially thesarne area as will become more apparent.

The area of the surface just cleaned and developed, then preferably, though not necessarily in all embodiments, advances through the field of pretransfer electrode 15 which recharges the surface of the drum in preparation for the transfer step. The surface carrying the developed image next advances into the transfer station 16 where the developed image is transferred to backing material 17, such as paper, and the paper supporting the transfer imageis then passed through a fixing station 18 comprising a heater or other suitable fixing means, the paper being transported by a belt 19. As the paper advances through the fixing station, the corresponding portion of the surface of the xerographic drum from which the image now supported on the paper was transferred, continues to advance through the cycle, but now supporting only the residual image of toner particles remaining after the transfer stop. In the apparatus of FIG. 1 after transfer, the drum surface advances past a negative charging device 21 wherein the charge on the drum is reversed in preparation for cleaning. The surface supporting the residual image continues to advance through a dischargestation 23 where the entire surface of the drum is flooded with light to discharge the photoconductive insulating layer. H

After discharge the surface of the drum is then ready to'be charged and exposed again, although the residual image from the previous exposure typically still remains on the same area of said surface. This is possible because the surprising and advantageous system of the present invention is used. In the previously known xerographic system, the drum surface would typically require an additional cleaning step before the charging and exposing steps of the subsequent xerographic cycle are performed. 1

In accordance with the present invention and as best shown in FIG. 2, two-component developer 24 material comprising carrier beads and electroscopic toner particles in triboelectric relationship is cascaded across the drum surface through a chute 26. Adjacent to the exit portion 27 of chute 26 is positioned an electrode 35 which is biased from a DC potential source 40 at a polarity opposite to that of the residual toner particles designated by numeral DC potential source 40 is variable for optimum operating conditions and may range from about 500 to about l,200 volts. It will be appreciated that the residual toner particles 42 are positively charged at transfer station 16 but that this charge is greatly lessened by a negative polarity by charging device 21 such that the residual image particles are only slightly positive as they approach positive charging device 12 as indicated by x in FIG. 2. After recharging the residual toner particles indicated by y are now highly charged positive, and after exposure enter the development zone. At the commencement of development they are physically contracted by the cascading developer material 24 and loosened from the drum surface. As a result the loose positive residual toner particles come under the influence of the field emanating from electrode 35 and become electrostatically attracted to the electrode as indicated by 2. In time, the residual toner particles building up on electrode 35 are knocked off by the cascading developer material 24 and become intermixed with the developer material with the toner particles clinging to denuded carrier beads. It has been found that by intermixing the newly cascaded developer material material 24 with the residual images of toner particles remaining on the drum surface, the latter are recharged negative thereby becoming reusable developer material.

In the past the residual toner particles had to be physically removed at a separate cleaning station or if simultaneous development and cleaning were attempted, copy quality would drop off rapidly in time. By the present invention, the xerographic reproduction system is more efiicient in that the removal of the residual image and its subsequent reuse in the system is effectuated. It should be readily appreciated how this invention contributes to the overall quality of copies as well as the efficiency of the copier system.

It has also been found that the toner particle size can be a significant factor. Toner particle size affects the efficiency of the electrostatic transfer of toner to latent electrostatic images and the transfer of residual toner from the xerographic plate back to the carrier. It has been found that both processes become more efficient with larger toner particle sizes. At a given toner concentration, smaller toner particles tend to cover more of the surface of the carrier beads thereby leaving less free bead surface available for development-cleaning or ble to being physically knocked from the plate surface. It has therefore been found advantageous to use toners having a particle size distribution which contains minimal amounts of relatively small toner particles. Toner particles may be classified as to particle size in a classifier for fine dry powders such as the Sharples K8 Super Classifier, manufactured by the SharplesCompany, 424 West Fourth Street, Bridgeport, Pennsylvania. In the-Sharples scale, toner particles are measured in microns. Toners with particles of average size by number in the range of about 10 to about 20 microns, with negligible numbers of particles of size less than 5 microns, gives results preferred over those of average size in the range of about 4 to about? microns, with about 50 percent of the particles of a size less than 5 microns. Toners in both of the above ranges give development-cleaning efiiciencieswhich are preferred over those attainable with particles of average size in the range of about 2 to about 3 microns, with about 90 percent of the particles less than 5 microns in diameter. The smaller toner particles will still perform the development-cleaning, although the build up of toner film on the apparatus typically is accelerated.

Another parameter is toner concentration in the developer mixture. The concentration of toner affects developmentcleaning primarily in the development part of the process. The cleaning will go on, but if the toner concentration is too high,

the cleaned residual images will be redeveloped as quickly as they are cleaned. Hence, the limiting concentration at one end is development capability (i.e., sufiicient toner to develop electrostatic latent images) while the other end point is the limit of the cleaning ability of the system. These concentration limitations depend on the degree of quality of copy desired. Toner concentration is conveniently expressed in terms of mass. per unit surface area, said surface being the surface of the carrier particles or beads. The advantageous cascade development-cleaning system of the present invention produces satisfactory results in toner concentration ranges of about 0.1 to about 0.4 mg. of toner per sq. cm. of carrier surface. At toner concentrations lower than about O.l mg./sq. cm., development in extended unexposed areas of the image pattern still occurs, but image toner uniformity tends to fall off rapidly. At higher toner concentrations the ability to clean is reduced. The reduction in cleaning capability may in part be due to increases in the amount of residual toner retained as the residual image. It is also thought that there is increased redevelopment of the residual image at these higher toner concentrations. A preferred range of toner concentrau'on in the developer mixture is about 0.2 to about 0.3 mg./sq. cm. These concentrations indicate that it is most desirable to closely control the toner concentration, preferably by automatic means.

Problems related to higher toner concentration include toner impaction and toner agglomeration, which may greatly reduce image quality.

It has been found that the addition of small amounts of dry solid hydrophobic lubricants effectively controls toner impaction and agglomeration. Such lubricants include metallic salts of fatty acids such as zinc searate, and other materials such as colloidal pyrogenic silica particles such as Cab-O-Sol, available from the Cabot Corporation, or various mixtures of such materials. An extensive group of such lubricants is recited in copending application Ser. No. 702,306, filed Feb. 2, 1968 US. Pat. No. 3,552,850. A preferred range of concentration for the lubricant is in the range of about 0.1 to about 1 percent by weight of toner.

The other component in the developer is a granular material called carrier" which by mixing with the toner particles triboelectrically acquires charge of polarity opposite that acquired by the toner. Carrier granules may be any shaped solid particle from flat platelets to cubes to spherical beads. The carrier may be made of any suitable material such as glass, plastic, metal or other granular material. Carrier granules of average size in the range of about 30 to about scavenging. The smaller toner particles are also less suscepti- 1,000 microns perform satisfactorily. A preferred range of carrier particles size is in the range of about 100 to about 600 microns.

The advantageous system of the present invention is useful in any electrostatographic process having an electrostatic latent image support surface. In the preferred process, xerography, the electrostatic latent image support surface is the surface of a photoconductive insulating layer. Selenium in its amorphous form is found to be a preferred photoconductive insulating material for use in xerography because of its extremely high-quality image-making capability, relatively high light response, and capability to receive and retain charged areas at different potentials and of different polarity. Any suitable photoconductive insulating layer may similarly be used in the practice of the invention. However, it is found that the inventive system performs more satisfactorily if the electrostatic latent image support surface is quite smooth. Typical photoconductive insulating layers include: amorphous selenium, alloys of surface arsenic or tellurium with selenium, selenium doped with materials such as thallium, cadmium sulfide, cadmium selenide, etc., particulate photoconductive materials such as zinc sulfide, zinc cadmium sulfide, French process zinc oxide, phthalocyanine, cadmium sulfide, cadmium selenids, zinc silicate, cadmium sulfoselenide, linear quinacridones, etc., dispersed in an insulating inorganic film forming binder such as a glass or an insulating organic film-forming binder such as an epoxy resin, a silicone resin, an alkyd resin, a styrene-butadiene resin, a wax or the like. Other typical photoconductive insulating materials include: belends, copolymer, terpolymers, etc., of photoconductors and nonphotoconductive materials which are either copolymerizable or miscible together to form solid solutions and organic photoconductive materials of this type include: anthracene, polyvinylanthracene, anthraquinone, oxadiazole derivatives such as 2,5-bis-(p-amino-phenyl-l 1,3,4-oxadiazole; 2-phenylbenzoxazole: and charge transfer complexes made by complexing resins such as polyvinylcarbazole, phenolaldehydes, epoxies, phenoxies, polycarbonates, etc., with Lewis acid such as tetrachlorophthalic anahydride; 2,4,7-trinitrofluorenone; metallic chlorides such as aluminum, zinc or ferric chlorides; 4,4-bis (dimethylamino) benzophenone; chloranil; picric acid; l,3,5-trinitrobenzene; l-choloroanthraquinone; bromal; 4- nitrobenzaldehyde; 4-nitrophenol; acetic anhydride; maleic anhydride; boron trichloride; maleic acid, cinnamic acid; benzoic acid; tartaric acid; malonic acid and mixtures thereof.

In addition to the advantageous use of the inventive system for simultaneously developing and cleaning an electrostatic latent image support surface, it is clear that the system of the present invention may also be used as a separate cleaning system. Thus, a one-pass cleaning system using developer as the functional cleaning medium, also shows that the advantageous development-cleaning system of the present invention can be used as both a development system and a cleaning system, in any two-cycle electrostatographic process. In such two-cycle processes, the development occurs during the first cycle and the cleaning occurs during the second cycle, which cycle is solely for the purpose of removing residual toner images from the electrostatic latent image support surface. Unlike the dual-station system described in the preceding paragraph, the two-cycle system achieves all of the objects of the preferred system, except that the recycling may involve slightly more complicated mechanisms and electrical circuits.

Although the description of the preferred embodiments of the invention system has been primarily directed to the use of the invention system in a xerographic process, it is appreciated and intended that the advantageous system of the present invention be incorporated in any suitable electrostatographic process.

Although specific components and proportions have been stated in the above description of the preferred embodiments of the development-cleaning system, other suitable materials and variations in the various steps in the system as listed herein, may be used with satisfactory results and various degrees of quality. In addition, other materials and steps may be added to those used herein and variations may be made in the process to synergize, enhance or otherwise modify the properties of the invention. For example, various photoconductive materials may be used in xerographic plates, and various photoconductor thicknesses may require somewhat different parameter setting for preferred results.

It will be understood that various other changes in the details, materials, steps, and arrangements of parts which have been herein described and illustrated in order to explain the nature of the invention, will occur to and may be made by those skilled in the art, upon a reading of this disclosure, and such changes are intended to be included within the principle and scope of this invention.

What is claimed is:

1. In an improved xerographic process comprising uniformly charging the moving surface of a photoconductive layer to a first polarity, exposing the charged surface to a lightand-shadow pattern to form an electrostatic latent image,

developing visual patterns from said electrostatic latent image by cascading a two-component developer material including carrier beads and smaller toner particles triboelectrically arranged with the toner particles having a polarity opposite to said first polarity across the moving photoconductive surface,

transferring said toner particles in the image areas to a support material by applying a charge of a polarity opposite to that of the toner particles thereby leaving residual toner particles remaining on said surface, the improvement comprising recharging said residual toner particles and surface layer to said first polarity, then moving said residual toner particles and surface layer past an electric field ranging from about 500 to 1200 volts DC at a second polarity opposite to the residual toner particles and at an interval time when commencing a pass through the development zone to cause the residual toner particles to be removed from the photoconductive surface, and passing the cascading developer material over the removed residual toner particles to cause intermixing therewith, and continuing with the aforementioned steps. 

